Recently, the discovery of oxide superconductors having a critical temperature which is higher than liquid nitrogen temperature made application technique of superconductors popular, and there is a fierce competition on the development of materials showing stable superconductivity characteristics under high temperature.
Especially, the Y--Ba--Cu--O system superconductors of the 123 system oxide (oxide system material wherein the mol ratio of Y:Ba:Cu is 1:2:3) have succeeded in achieving a high critical current density by a unique method. The development of applications to bearings, flywheels, carrying devices and the like are thought of using the power of the "superconductor that can create a large electromagnetic power by the mutual operation with a magnetic field".
Further, other than Y123 system oxide superconductors, advantageous superconductivity characteristic are recognized in RE123 system oxides having a composition of various rare-earth elements (RE) instead of Y (yttrium). Research is going on for these oxides for the development of better superconductivity application equipment.
Conventionally, "flux method", "melt solidification method" and the like were used to manufacture Y123 system oxide superconductors and RE123 system oxide superconductors.
The "flux method" is a method of creating an oxide superconducting crystal from a solvent under its solubility limit by lowering the temperature continuously of a supersaturation solution having a relatively uniform material composition (flux: solvent) formed of a mixture of oxides. The "melt solidification method" is a method of creating an oxide superconducting crystal by heating the material composition (flux) to a temperature above the peritectic temperature of the object oxide in order to create a mixture of solid-phase and liquid-phase material, and then lowering the temperature continuously, provoking peritectic reaction.
Further, various single crystal growing methods such as "FZ method (floating zone melt method)", "unidirectional solidification method" and "crystal pulling method" are tested for application, and attention is given to the "crystal pulling method" which is advantageous for mass production.
This "crystal pulling method" is a representative method for manufacturing a single crystal continuously. The method manufactures an oxide superconducting single crystal by immersing seed crystal to the solution surface of a material composition kept at a melted state inside a crucible, growing superconducting single crystal on the seed crystal by the peritectic reaction at the solution/grain interface and gradually and continuously pulling out the single crystal from the solution in order to gain an oxide superconducting single crystal.
FIG. 1 is an explanatory view showing the method of manufacturing a single crystal by the "crystal pulling method".
In order to manufacture, for example, a Y123 system oxide single crystal by the "crystal pulling method", a Y--Ba--Cu--O system oxide (the crystal of a high-temperature-phase Y.sub.2 Ba.sub.1 Cu.sub.1 O.sub.5 (Y211)) 2 is inserted to an yttria crucible 1 as a supply source of a solute Y (the material of the crucible not limited to yttria but any material such as magnesia, alumina, stabilized zirconia and the like of refractory material having resistability could be utilized as the crucible), and filling the area above it with a "solution 3 of a solvent mixing and pre-baking barium carbonate and copper oxide so that the mol ratio of Ba:Cu would be 3:5 (Ba/Cu=0.57)" and keeping the liquid-phase surface temperature between 960-1010.degree. C., bringing a seed crystal stick 4 of a refractory material close to the liquid-phase surface, and contacting a thin film (seed crystal) 5 of Y123 grown using plasma evaporating method to a magnesia single crystal which will act as the seed of crystal growth fixed to the end of the seed crystal stick. Next, the seed crystal stick 4 is rotated in a speed of approximately 100 rpm and pulled to the upper direction by a speed of about 0.2 mm/hr, pulling out continuously a YBa.sub.2 Cu.sub.3 O.sub.7-x single crystal 6 grown using the YBa.sub.2 Cu.sub.3 O.sub.7-x crystal 5 as the seed.
The method of manufacturing a RE123 system oxide single crystal by a "crystal pulling method" is almost the same as in the above case of manufacturing said Y123 system oxide single crystal. However, in this case, the aimed RE system oxide is used as the seed crystal, and normally as the supply source of RE (rare-earth element) solute, either a RE422 (RE.sub.4 Ba.sub.2 Cu.sub.2 O.sub.10) or a RE211 composed RE--Ba--Cu--O system oxide is inserted to the crucible, or a RE.sub.2 O.sub.3 crucible is used to supply RE solute.
However, the 123 system oxide superconductors manufactured by the above described methods show a relatively fine superconductivity by the Y123 system, but there was a problem that the RE123 system superconductors did not show very high critical temperature.
The reason for such phenomenon was thought to be that the chemical composition of the gained RE123 crystal was excluded from the composition of 123 phase with a value of x being 0 to 0.05, since the ionic radii of rare-earth elements (RE) was relatively large and thus close to the ionic radii of Ba, resulting in the occurrence of the mutual substitution of RE and Ba at the time of creating a superconductivity phase by cooling/solidifying the melted material.
FIG. 2 is a "1/2RE.sub.2 O.sub.3 --BaO--CuO ternary system status view" showing oxides under the atmospheric environment including RE (La, Nd, Sm, Pm, Eu, Gd and the like) having a relatively large ionic radii. As is shown in FIG. 2, a "solid solution zone having width" exists along the line extending from the RE123 phase to the upper right direction. This solid solution zone exists because under atmospheric ambiance, RE.sub.1+x Ba.sub.2-x Cu.sub.3 O.sub.y (x&gt;0, 6.0&lt;y&lt;7.2) phase becomes stable shifting from the RE123 phase. As was described, and as could be understood from the existence of said solid solution zone, in an oxide including RE having a relatively large ionic radii, the mutual substitution of RE and Ba occurs when solidifying and growing superconductivity phase under atmospheric ambiance.
As is shown in FIG. 3, the superconductivity of the RE123 system oxide superconductor changes according to the amount of substitution x of RE and Ba, wherein the larger the amount of substitution x, the lower the critical temperature, and in contrast, the smaller the amount of substitution x (that is, with a composition close to RE123 oxide), the higher the critical temperature.
As was explained above, one cause of the prevention of the achievement of high critical temperature was the mutual substitution between RE and Ba occurring at the step of growing a superconducting phase by cooling and solidifying the melted material (the step of nucleus creation and growth).
Then, an "OCMG method (method of melt solidification under low oxygen ambiance)" was suggested wherein the amount of oxygen included in the ambiance is controlled to be low when the melt and solidification of the material flux is performed to grow crystals.
However, the 123 system oxide superconductors created by this OCMG method is accompanied by a very disadvantageous manufacturing problem, because even though a RE123 with high critical temperature could be gained with small amount of substitution having a narrow width of the amount of Ba substitution under low oxygen partial pressure ambiance, it is disadvantageous that it has to be manufactured under a controlled low oxygen ambiance.
Therefore, the object of the present invention is to provide a method of manufacturing a RE123 system oxide superconductor under atmospheric ambiance without having to control the ambiance, showing a great superconductivity with a very small amount of mutual substitution between RE and Ba, having a critical temperature of higher than 90 K and with a narrow superconducting transition width .DELTA.T.